Method and material for producing phosphate coating



Patented May 21, 1935 UNITED STATES METHOD AND MATERIAL FOR PRODUCINGPHOSPHATE COATING John S. Thompson and Van M. Darsey, Detroit, Mich.,assignors to Metal Finishing Research Corporation, Detroit, MichiganMich., a corporation of v No Drawing. Application March 27, 1933, SerialNo. 662,998

11 Claims.

This invention relates to an improvement in the production of phosphatecoatings, and particularly to the production of a coating on iron orsteel adapted to bond paint or the like to 6 the metal.

The production of phosphate coatings on iron and steel by means of adilute phosphate bath has been common for many years. A comparativelyrecent discovery is that the formation of the coating can be verymaterially expedited by including in the treating solution sodiumnitrate or an equivalent material. Where the treatment is by immersingthe articles in a bath, the continued addition of sodium nitrate toreplenish the bath sometimes unduly builds up the sodium in the bath sothat the bath becomes so low in phosphates of iron or manganese as tomake it ineffective. For this reason the addition of a nitrate of amaterial that will either enter into the coating, or form an insolublecompound with the ingredients of the solution and so be precipitatedout, or will be volatilized and so driven off, is preferable forreplenishing the bath. In other words, it is desirable to add a nitratewhich does not introduce a material that builds up deleteriously in thebath.

It is also desirable to have a high manganese content in the bath, asmanganese phosphate forms a very desirable coating upon the metal.

A specific example of the formation and replenishment of a bath will bedescribed next, and then some principles upon which the modificationsmay be based will be set forth.

Sodium nitrate is readily available and is suitable for use withphosphates for starting a bath. Varying amounts of the sodium nitratemay be mixed with phosphates and dissolved in water. One example ofsuitable proportions is to mix 110 lbs. of sodium nitrate with 115 lbs.of manganese dihydrogen phosphate, although the proportion of sodiumnitrate to the phosphate may be varied widely. About 19 grams of coppercarbonate may be added, if desired. From 35 to 50 lbs. of the mixturedissolved in 100 gallons of water produces a solution which coats workintroduced therein very quickly and evenly. Although the copper is notnecessary, the coating is expedited by the inclusion of the small amountof copper, and it also aids in producing a coating action upon surfaceswhich are somewhat difficult to attack for any reason.

After a bath has been started in this manner, it may be replenished byadding manganese phosphate, containing copper if desired, and manganesenitrate. One-fourth of a pound of copper carbonate to 100 lbs. of themanganese phosphate has been employed with success in replenishing abath of the kind described. Any less amount of copper may be employed,and some more might be used, but too much copper produces an undesirablecoat. The manganese nitrate and manganese dihydrogen phosphate shouldnot be mixed and allowed to stand before being introduced into the bath,as there is reaction between the nitrate and dihydrogen phosphate whichrapidly reduces the efficiency of the powder.

Manganese nitrate readily absorbs moisture from the air and is diflicultto handle for that reason. By adding normal manganese phosphate ormono-hydrogen manganese phosphate to the manganese nitrate, thedifliculty in this respect is largerly avoided, and at the same time themanganese content of the bath increased, and no undesirable material isintroduced into the bath. No injurious reaction takes place between thenormal or mono-hydrogen manganese phosphate and the manganese nitrate.

The normal or mono-hydrogen manganese phosphate may be added to thenitrate in widely varying proportions but two parts of the phosphate tothree of the nitrate have been found to work very well. When thephosphate and nitrate are mixed in this proportion, the powder keeps andmay be transported and handled with fair facility. Replenishing the bathwith equal parts of this mixture and manganese dihydrogen phos-' phatemaintains a bath which operates quite satisfactorily, some copper beingusually included with the phosphate.

Where the Bath is started with manganese dihydrogen phosphate andmanganese nitrate. without the use of\any sodium nitrate, and isreplenished with thexsame materials, the new bath operatessatisfactorily at first, but quickly deteriorates so that it isunsatisfactory, although the presence of copper mitigates thediiflculty. Where the bath is started with a mixture of sodium nitrateand manganese dihydrogen phosphate and is replenished with the samemixture. the initial operation is satisfactory and the work continues tobe satisfactory for a considerable time, but the sodium gradually buildsup in the bath and the manganese phosphates and iron phosphates arereduced in quantity until the bath becomes ineffective. By starting thebath with the mixture of manganese dihydrogen phosphate and sodiumnitrate and maintaining the bath by the addition of manganese phosphatesand.manganese nitrate, ashescribed, the bath is maintained at highefficiency, but in all cases some copper increases the efilciency.

By proceeding in the described manner, the phosphates of iron andmanganese are not displaced by sodium phosphate to an injurious extent,nor do they become undesirably high in proportion to the otheringredients of the bath, and the solution is maintained in properbalance so that a good paint-holding coat high in manganese phosphate isproduced very quickly.

While manganese nitrate is preferred in the replenishment of the bath,for reasons given above, it will be evident that some use is possible ofnitrates of metals such as magnesium, calcium, barium, strontium, zinc,cadmium and iron, which enter into coating as phosphates.

Various changes may be made within the scope of the appended claims.

What we claim is:-

1. A process of coating iron or steel which consists in forming asolution containing acid phosphates and a nitrate of an alkali metal,immersing in the solution articles having surfaces of iron or steel, andmaintaining the strength of the solution by adding thereto acidphosphates and a nitrate of an element of the class that consists ofmagnesium, calcium, barium, strontium, manganese, zinc, cadmium andiron, which enter into the coating as bases of phosphates.

2. A process in accordance with claim 1, and in which a copper compoundis dissolved in the solution.

3. A process of coating iron or steel which consists informing asolution by dissolving in water acid phosphates and sodium nitrate,heating the solution and immersing therein articles having surfaces ofiron or steel, and replenishing the solution by adding thereto acidphosphates and nitrate of manganese.

' phate coating bath by the addition of materials including dihydrogenphosphate of manganese,

manganese nitrate, and phosphate of manganese less acid than thedihydrogen phosphate.

8. In coating surfaces of iron or steel, the method which consists inreplenishing a phosphate coating bath by the addition thereto of onemixture consisting chiefly of phosphate of manganese, less acid than ismanganese dihydrogen phosphate, and nitrate of manganese, and anothermaterial consisting essentially of manganese dihydrogen phosphate.

9. A method in accordance with claim 8, and in which a compound ofcopper is included in one of the mixtures.

10. A method in accordance with claim 8, and in which the proportions ofthe three named, chemicals is approximately 2, 3 and 5 respectively.

11. A material, for use in replenishing a phosphate bath, consistingessentially of manganese nitrate and material of the class consisting of95 manganese mono-hydrogen phosphate and nor"- mal manganese phosphate.JOHN S. THOMPSON,. VAN'M. DARSEY. I

